One known method of coating a metal substrate comprises hot dipping steel strip in a molten bath of zinc and aluminium to form a coating of a zinc/aluminium alloy. This method is known as the ZINCALUME® process. Silicon (at around 1.5 wt %) may be added to the bath to prevent excessive alloying between the molten coating metal and the steel strip. In the absence of silicon, such alloying may occur so rapidly that the resultant heat generated can actually heat up the strip. The resultant coatings in the absence of silicon may therefore develop a thick (>20 μm), brittle Al—Fe—Zn alloy coating layer, rendering the product effectively useless. When silicon is present in the bath, a thin (˜1 μm) Al—Fe—Zn—Si quaternary alloy layer can form, which acts as a blanket on the steel surface to separate the surface from the molten coating metal, thereby preventing rapid alloying and allowing a sufficient coating overlay to form.
However, the presence of silicon in the bath introduces other problems. For example, it has been observed that, during coating solidification, silicon needles precipitate out and grow within the coating. These silicon needles are often connected to the Al—Fe—Zn—Si quaternary alloy layer and can provide a corrosion pathway to the alloy layer. The silicon needles can also increase the brittleness of the coating layer. Also, the presence of silicon in the bath means that certain other bath additives, such as magnesium, may not be able to be used to produce desirable coating properties. For example, silicon reacts with magnesium to produce a brittle Mg2Si phase, which in turn increases the brittleness of the coating layer.
The above description is not to be taken as an admission of the common general knowledge.